Current state-of-the-art engine controls rely almost exclusively on exhaust gas sensing to maintain the engine air-fuel ratio at a value that minimizes exhaust emissions. However, such sensors typically require heating for a significant period before the sensor is useful for control following a cold start. For this reason, engine spark timing and fueling during cranking and warm-up is generally performed based on an open-loop calibration. For a given amount of fuel delivered to the intake manifold, the air-fuel ratio delivered to the cylinder varies considerably for fuels of different volatility. The fuel in the cylinder comes in part from vaporized fuel from the current injection event and in part from fuel vaporized from the port walls wetted by previous injections. The rate at which both these components vaporize depends not only on temperature and pressure, but also on the fuel volatility, which may vary considerably from tank to tank.
The above-described dependence on fuel volatility is illustrated in FIG. 1A, where the solid and broken traces represent the fuel vapor-to-air equivalence ratio .phi..sub.v as a function of time, as measured in the engine exhaust stream during a cold start test. The solid trace depicts the ratio .phi..sub.v for a fuel having a relatively high volatility, while the broken trace depicts the ratio .phi..sub.v for a fuel having a relatively low volatility. As seen in the graph, the ratio .phi..sub.v is roughly 20% richer with the high volatility fuel than with the low volatility fuel. FIG. 1B shows the indicated mean effective pressure (IMEP) for the two fuels; again, the solid trace corresponding to the high volatility fuel, and the broken trace corresponding to the low volatility fuel. The IMEP parameter, a measure of the work produced, is significantly lower and more variable for the low volatility fuel than for the high volatility fuel. This is a result of the low volatility fuel yielding a leaner mixture, which for a given spark timing, burns later during the engine cycle than the richer mixture. For the condition depicted, these later burning cycles release heat further after top-dead-center for the piston, resulting in less useful work being produced with nominal spark advance settings.
The uncertainty in .phi..sub.v delivered to the cylinder and the appropriate spark timing forces design engineers to enrich the cold calibration to insure that operating with low volatility fuel will not result in driveability problems. This enrichment to compensate for low volatility fuels causes .phi..sub.v to be richer-than-optimum with high volatility fuel, resulting in higher hydrocarbon emissions than if the appropriate calibration for that fuel was used. Similar phenomena occur, although to a lesser degree, during fueling transients. Thus, it is apparent that differences in fuel volatility adversely affect both emissions and performance with conventional control strategies.
Accordingly, what is needed is a method for detecting fuel volatility in order to deliver fuel more accurately, for improved emissions and driveability.